(26) session 26 electrophilic addition of alkenes

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Session 26

Organic Chemistry, UNAM School of Medicine1

Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of

alkenesalkenesalkenesalkenes

Dr L.H.A. Prins (Ph.D.)

Dept. of Pharmacy

UNAM

Learning Outcomes

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� By the end of this session, the student should understand:

�Electrophilic addition of hydrogen halides to alkenes

�Addition of water to alkenes

�Addition of alcohol to alkenes

�Addition of hydrogen to alkenes

Organic Chemistry, UNAM School of Medicine

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Reactivity of alkenesReactivity of alkenesReactivity of alkenesReactivity of alkenes

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� Double bond of alkenes – consist

of a σ & a π bond

� ̟ bond is weaker than the σ bond

� ∴ ̟ bond is broken when an

alkene undergoes a reaction

� Alkenes = e−-rich, because ̟ bond consists of a cloud of e− above

& below ̟ bond

� ∴ Alkenes = nucleophiles & they can react with electrophiles


Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides

(H(H(H(H----X) X) X) X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation

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� All alkene reactions have one feature in common: Delocalised ̟

e− of the C-C double bond are attracted to an electrophile (E)

� Each reaction starts with addition of E to one of the sp2 carbons of

the alkene & concludes with addition of a nucleophile (Nu) to the

other sp2 carbon

� ∴ End result = ̟ bond breaks & sp2 carbons form new σ bonds

with the E & Nu


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(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation

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� Example:� Electrophilic addition reaction of HBr to but-2-ene

� Step 1: Relatively slow addition of proton, (H+) (electrophile) to the

alkene (nucleophile) to form a carbocation intermediate

� Step 2: + charged carbocation intermediate (electrophile) reacts

rapidly with − charged bromide ion (nucleophile)




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� Electrophilic addition reaction of alkenes make them an

important class of organic compounds, because they can be used to

synthesise a wide variety of other compounds

� Product obtained depends on electrophile & nucleophile used in the

addition reaction

� If electrophilic reagent that adds to an alkene is a hydrogen halide

(HF, HCl, HBr or HI) – reaction product will be a halo-alkane

(alkyl halide)


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� Examples:

� Because alkenes above have the same substituents on both sp2 carbons, it

does not make any difference which sp2 carbon the E & Nu attach to

� ∴ Same product will be obtained in either case

� But what happens if alkene does not have the same substituents on both

sp2 carbons?




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� Example:

� When reaction is carried out, only product isolated is tert-butyl

chloride (product 1)

� Step 1 of reaction (addition of E (H+) to an sp2 carbon ) = rate-

determining step!

� ∴The particular carbocation that is formed faster determines final

product of reaction


HCl orHCl

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� Why is tert-butyl carbocation formed faster than isobutyl carbocation?

� Answer lies in the stability of the carbocation

� Carbocation classification - according to # of alkyl substituents bonded

to + charged C

� Carbocation stability ↑ as the # of alkyl substituents bonded to +

charged C ↑


� tert-butyl cation (a 3° carbocation)� isobutyl cation (a 1° carbocation)

RegioselectivityRegioselectivityRegioselectivityRegioselectivity of electrophilic addition of electrophilic addition of electrophilic addition of electrophilic addition

reactionsreactionsreactionsreactions

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� In both cases: More stable 3° carbocation formed more rapidly than less stable 2° carbocation

� ∴ Major product of each reaction = one that results from forming the 3°carbocation


Hl

Major product Minor product

Major product Minor product

HBr

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� The 2 products of each of these reactions are structural isomers of

one another

� Reaction in which 2 or more structural isomers could be obtained as

products, but 1 of them predominates (obtained in larger amounts

than the other products), is called a regioselective reaction

� Regioselectivity = the preferential formation of one structural

isomer over the other




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� Addition of HBr to 2-pentene is not regioselective:

� Addition of H+ to either sp2 carbons produces a 2° carbocation, both carbocation intermediates have the same stability, so both will be

formed equally easily� ∴∴∴∴Approximately equal amounts of the two alkyl halides will be

formed


HBr

2-bromopentane50%

3-bromopentane50%

2-pentene

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Markovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s rule

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� By examining all addition reactions of hydrogen halides to alkenes we

have seen so far, we can devise a rule that applies to all electrophilic

addition reactions to alkenes: the electrophile (H+) adds to the sp2

carbon that is bonded to the greater number of hydrogens

� This rule is referred to as Markovnikov’s rule


HCl

H+ will preferentially add to C-1 & NOT to C-2!!

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Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H2222O O O O ---- HydrationHydrationHydrationHydration

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� When H2O is added to an alkene, no reaction takes place

� Reason: O–H bonds of H2O are too strong – water is too weakly acidic –

to allow the H to act as E to start the reaction by adding to the

nucleophilic alkene

� If an acid (ex. HCl or H2SO4) is added as a catalyst, a reaction will

occur

� Reason:The acid provides an electrophile (H+)

� Product of the reaction is an alcohol

� Hydration = addition of H2O to any molecule


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� First 2 steps of mechanism for acid-catalysed addition of H2O to alkene -

essentially the same as first 2 steps of mechanism for addition of

hydrogen halide to alkene. Results in formation of a protonated alcohol

� Protonated alcohols = very strong acids (proton donors)

� ∴ In 3rd step of reaction, protonated alcohol loses a proton (H+) to form

the final product, an alcohol



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� A H+ (donated from HCl or H2SO4) adds to alkene in 1st step & a

H+ is returned to reaction mixture in final step

� ∴ H+ is not consumed in the reaction

� The acid acts as a catalyst in the reaction – this is an acid-

catalysed reaction


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Electrophilic Electrophilic Electrophilic Electrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)

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� Alcohols react with alkenes in the same way that H2O does

� As with addition of H2O, addition of alcohol requires an acid catalyst

� Product of reaction = ether

� Mechanism for acid-catalysed addition of alcohol is essentially the

same as mechanism for acid-catalysed addition of H2O

� Only difference: Nu is ROH instead of H2O


HCl

Electrophilic Electrophilic Electrophilic Electrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)

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� NB!!! - Do not memorise the products of alkene addition

reactions

� Know the reaction mechanism

� When presented with a reaction, ask yourself: “Which is the

electrophile?” & “Which is the nucleophile?”


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Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H2222) ) ) ) ----

HydrogenationHydrogenationHydrogenationHydrogenation

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� In the presence of metal catalyst such as Pt (platinum) or Pd

(palladium), hydrogen (H2) adds to = bond of alkene to form an

alkane

� Without metal catalyst, reaction will not take place

� Reason: H–H bond is very strong & energy barrier is enormous

� Catalyst ↓ activation energy by breaking the H–H bond

� Pt & Pd are used adsorbed* on carbon (Pt/C, Pd/C)*Adsorption = adhesion of atoms to a surface


Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H2222) ) ) ) ----

HydrogenationHydrogenationHydrogenationHydrogenation

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� Examples:

� Addition of H2 to an alkene is called alkene hydrogenation & since

the reaction requires a catalyst, it is specifically called a catalytic hydrogenation of an alkene

� Details of mechanism for catalytic hydrogenation are not yet completely

understood


H2

Pd/C

H2

Pt/C

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Quiz: Electrophilic addition to alkenes

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1. w

2. w


Quiz: Electrophilic addition to alkenes

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3. W

4. w


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Thank you

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END


(26) session 26 electrophilic addition of alkenes

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